When you are using a real 24-bit display Gnome shell (probably via libmutter) currently displays stippled and striped patterns on the root window (desktop background) and title bar of focused window like this:
Various other visual elements are also distorted in this way.
$ lsb_release -d
Description: Fedora release 18 (Spherical Cow)
Unfortunately, how to determine the true bitmap depth is difficult. The only way I currently know is to try running X with the -pixmap32 option and see if it fails.
yum install xterm
systemctl stop gdm
xinit -- -pixmap32
Fatal server error:
[ 477.402] Screen(s) can't use the required depth 24 pixmap format (32). Exiting